High-temperature behavior of axinite-(Mn), kornerupine and leucosphenite

In situ high-temperature powder X-ray diffraction (HTPXRD) and differential scanning calorimetry (DSC) of three natural borosilicates have been performed in the temperature range of 25—1200 °С. Axinite-(Mn) melts incongruently at 900 °С forming anorthite and bustamite. Leucosphenite decomposes at 850 °С to fresnoite and cristobalite. According to DSC data, kornerupine decomposes at 1177 °С and sapphirine, indialite, and spinel were observed as the products of kornerupine heating. The calculation and orientation of thermal expansion tensor have been performed using HTPXRD data. The study showed that the borosilicates expand weakly and almost isotropically. The average volumetric thermal expansion coefficients are 21.3, 22.7, and 32.9 ∙ 10^–6 °C^–1 for axinite-(Mn), kornerupine, and leucosphenite, respectively. Leucosphenite has a maximum volumetric expansion most likely due to the pronounced layered character of the crystal structure. The least symmetric structure of axinite-(Mn) has the maximal anisotropy of thermal expansion in the temperature range of 600—900 °C.